Digital encoding and transmission techniques are widely used in today's telecommunications systems for transmitting information. For example, the Global System for Mobile communication (GSM) is a digital system presently in widespread use, and the Enhanced Data rates for GSM Evolution (EDGE) is a digital system which is gaining popularity. In such telecommunications systems, the information to be sent is encoded or modulated onto a radio frequency signal for transmission to a receiver. Problems often arise in decoding or demodulating the signals at the receiver end due to channel distortion of the signal during transmission. That is, the transmitted signal is subject to distortion in the air interface, or radio channel, between the transmitter's antenna and the receiver's antenna.
Examples of such signal distortions include multipath propagation, fading, or other electromagnetic disturbances. Multipath propagation occurs when one portion of a transmitted signal takes a direct route between the transmitting antenna and the receiving antenna, while other parallel portions of the signal take indirect routes. The multiple signal paths are often caused by the signal being reflected off of a building or other object nearby or in between the transmitter or receiver. Since it takes longer for a signal to traverse an indirect path than the direct path, the parallel portions of the signal traveling along the multiple paths arrive at different times, thus interfering with each other. The distortion due to multipath propagation is often referred to as Inter-Symbol Interference (ISI).
Conventional receivers generally process a received signal using a training sequence to compensate for the assumed distortions of the channel. A training sequence is a predefined digital string which is typically sent along with data transmissions at regular time intervals. For instance, training sequences are transmitted as part of the burst transmissions in the aforementioned GSM and EDGE systems. A connection in a GSM system typically uses any of eight different predefined training sequences.
The training sequence is used to determine the timing position in order to synchronize the received signal and correct for time delays. Upon determining the timing position, the training sequence is used to estimate parameters, such as filter tap coefficients, for use in the receiver's channel model (i.e., a mathematical representation of the channel). This channel estimate is used to compensate for delay and attenuation characteristics, or other channel distortions which may cause ISI.
FIG. 1 is a block diagram of a portion of a conventional receiver assembly 100 which includes conventional synchronizer, channel estimator and equalizer units. The conventional receiver assembly 100 may be adapted for use in a digital telecommunication system such as a GSM system or an EDGE system. A radio signal received at antenna 110 is down converted and low pass filtered in radio receiver unit 112 which, in turn, produces baseband signal (yt). The receiver unit 112 may, for example, be a conventional homodyne receiver as discussed in RF and Microwave Circuit Design for wireless Communications by E. Larson (Artech House Inc., Norwood, Mass., USA, 1996).
In order to synchronize the data transmissions of a received signal, the training sequence which will be used in the data transmissions must be determined prior to beginning the data transmissions. For example, the training sequence may be specified by a base station in communication with a mobile station, or may be determined through a negotiation process between the base station and the mobile station. That is, the training sequence may be provided to the receiver assembly 100 through control channel signaling or by some other way of communicating it between the base station and mobile station upon establishing a connection.
Under ideal conditions a received training sequence should closely match the known training sequence TS. In practice however, various distortions introduced by the air interface affect the transmitted signal. To detect a received training sequence, the portion of the received baseband signal burst containing the training sequence is retrieved from a memory of the system (not shown) and supplied to synchronization unit 114. The synchronization unit 114 correlates different portions of the received signal with the known training sequence (TS) in order to find the synchronization position (i.e., the start position of the training sequence within the received signal).
The synchronization information and the received signal are provided to channel estimator unit 116. The channel estimator unit 116 assumes a predetermined channel model, and computes the K:th order channel filter model coefficients {hi}i=O,K on the basis of the received signal and the known training sequence TS. The number of channel taps, K, is typically specified from the maximum expected delay spread for the radio signal of the telecommunication system in use. The channel filter model coefficients, synchronization information and the received signal are then fed to an equalizer unit 118, which may, for example, be a Viterbi equalizer. The output of the equalizer unit 118 is the decided symbol (ût). The output information from the equalizer unit 118, that is, the decided symbol (ût), may then be used in further processing the received signal.